This invention relates to a circuit employing a semiconductor switching element for maintaining the power supply in a camera (hereinafter referred to as "a power supply maintaining circuit," when applicable).
A conventional power switch in a camera is mechanically operated that is, it is turned on by operation in association with the release button or the winding lever or a special actuation member. Some power switches are mechanically held closed for light measurement even if they are released after operation. However, this technique is disadvantageous in that, if the camera operator forgets to turn off the switch or restoring the related operating member, the battery will be unnecessarily consumed. This difficulty is multiplied because, the camera when stored will continue to drain the battery and the camera may be unusable without new batteries if the period of storage is even of moderate duration.
This disadvantage may be overcome by a method in which the power switch is coupled to a timer so that it is turned off after a predetermined period of time. Typically, the circuit according to this technique comprises a charging circuit having a capacitor and a resistor forming a timer with a given time constant, a switching element for controlling the charging circuit, and a transistor connected in series to a shutter control circuit or a display circuit. The transistor is maintained conductive (ON) until the voltage across the capacitor in the charging circuit reaches a predetermined value. The transistor is then switched (OFF) and the charging circuit is opened.
In this case, voltage drop takes place in the transistor which is a semiconductor switching element. The supply voltage of the circuit is accordingly decreased. Since in this technology requirements for a low voltage electric source has been a design criteria in contemporary cameras, the aforementioned voltage drop of the semiconductor switching element is significant. If elements such as magnets consuming a large current are employed in the shutter control circuit together with a display circuit, the base current of the semiconductor switching element, namely, the transistor is increased. Therefore if the time constant of the charging circuit is to be maintained unchanged, it is necessary to increase the capacitance of the capacitor. This technique is therefore difficult to use in a practical circuit in a contemporary camera design having large power requirements.
In view of the foregoing deficiencies in the prior art, a circuit according to this invention comprises a power switch operated in association with a release button and is connected in parallel to a semiconductor switching element. The power switch triggers a timer. Therefore, current is supplied to the semiconductor switching element only for the light measurement period during which the quantity of current consumption is relatively small. The power switch is, of course turned on during the exposure. Hence, even if current is supplied to a circuit element such as a magnet which consumes a large current, the voltage drop will not take place as in the case of the prior art, since the semiconductor switching element is shortened.
Accordingly, the amount of current driving the semiconductor switching element can be reduced, and the capacitance of the capacitor can be reduced yet achieve the same time constant. This is a significant advantage in installing the circuit in a camera.
Accordingly it is an object of this invention to provide for a circuit that minimizes power consumption in a camera.
It is another object of this invention to provide for a circuit used in cameras with a shutter control circuit or display that provides a direct timing circuit maintained in parallel between those power consuming elements to reduce transistor switching voltage drops during exposure.
These and other objects of this invention will become apparent from the drawings and the description of the preferred embodiment that follow.